Method of electroplating and pre-treating aluminium workpieces

ABSTRACT

Disclosed is a method of applying a metal layer onto at least one surface of an aluminium or aluminium alloy workpiece, including the steps of pre-treating the surface by cathodic activation in a pre-treatment bath containing sulphuric acid and metal-ions selected from the group consisting of nickel, iron and cobalt, and applying a metal layer by electroplating the pre-treated workpiece. The metal layer is selected from the group consisting of nickel, iron, cobalt, and alloys thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims the benefit of U.S. provisional patent application No. 60/644,530, filed Jan. 19, 2005 and European patent application number EP-05075082.7 filed Jan. 19, 2005, both incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a method of applying a metal layer onto at least one surface of an aluminium or aluminium alloy workpiece or article, comprising the steps of a simple pre-treating step of cleaning and activating the surface and yet producing good adhesion of the subsequently applied metal layer. The invention also relates to an aluminium alloy product plated on at least one surface with a metal layer. More specifically, the invention relates to a method of applying a metal layer of a braze-promoting metal onto the clad layer of an aluminium alloy brazing sheet product to be used in a fluxless brazing operation.

As will be appreciated herein below, except as otherwise indicated, all alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association.

BACKGROUND OF THE INVENTION

Nickel plating of aluminium products is widely used because nickel provides a bright, shiny appearance, is long lasting and can conduct electricity. Another, more particular use of nickel plating is made in the manufacture of brazing sheet products. Aluminium alloy brazing sheets comprise an aluminium alloy core and a clad layer of filler alloy on one or both sides. Aluminium brazing sheets are widely used, e.g., in the production of heat exchangers.

However, the use of aluminium-silicon alloys as filler material is problematic because the aluminium oxide layer has to be disrupted during brazing. This may be effected by applying a chemical flux onto the workpiece before brazing. Fluxes for use in brazing aluminium alloys usually consist of mixtures of alkali and alkaline earth chlorides and fluorides or cryolite. The flux operates at the brazing temperature to disrupt, spread and dissolve the oxide film. However, applying the chemical flux onto the workpiece is a rather laborious and therefore expensive process.

In the past, fluxless brazing techniques have therefore been developed and employed as described for example in US2003/0098338-A1, incorporated herein by reference in its entirety. In one such technique, a braze-promoting metal of cobalt, iron, or more preferably nickel, is coated on a part to be brazed. During brazing, the nickel reacts exothermically with the underlying aluminium alloy, thereby disrupting the aluminium oxide layer and permitting the underlying molten aluminium clad metal to flow together and join. As this method does not require a fluoride flux, it is also suitable for utilization with magnesium-enriched aluminium alloys, such as are beneficially used in heat-exchanger constructions.

In addition to the nickel, iron or cobalt coating, a wetting agent may also be added in order to improve the wettability of the clad alloy during the brazing process. However, nickel plating requires extensive pre-treatment of the metal surface such as cleaning, etching, desmutting, etc. This is again due to the presence of the tenacious oxide layer. If the aluminium alloy surface has not been properly pre-treated, the nickel coating will either have poor adhesion, or will be contaminated and thereby impede the brazeability of the product. Therefore, nickel plating with all necessary pre-treatment steps is an expensive and environmentally unfriendly process.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for applying by electroplating a metal layer onto an aluminium alloy product, which method requires as few steps as possible, and does not require the use of fluoride containing components.

It is furthermore an object of the present invention to obtain a metal coated aluminium alloy product, wherein the applied metal coating adheres well and may serve to break up the oxide layer during a subsequent brazing operation.

The present invention solves one or more of these objects through a method of applying a metal coating and an aluminium alloy product.

The method of applying a metal layer onto at least one surface of an aluminium or aluminium alloy workpiece, comprises the steps of pre-treating the surface by cathodic activation in a pre-treatment bath containing sulphuric acid and metal-ions selected from the group consisting of nickel, iron and cobalt, and applying a metal layer by electroplating the pre-treated workpiece, and wherein the metal layer is selected from the group consisting of nickel, iron, cobalt, and alloys thereof. It will be immediately clear to the skilled person that when the applied metal layer contains nickel or an a nickel-alloy that the pre-treatment bath should contain nickel-ions, and where iron or cobalt or alloys thereof are being applied that the pre-treatment bath contains iron-ions and cobalt-ions respectively.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an angle-on-coupon for brazing tests.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is based on the finding that direct metal, for example nickel, plating of aluminium alloy products is possible after cathodic activation in a simple sulphuric acid solution to which only nickel ions, e.g. in the form of nickel sulphate, has been added. No fluoride components are needed in this activation process. Because the activation bath contains the same ingredients as a Watts bath which is preferably used as nickel plating bath, cross-contamination is excluded. Also, no problems in effluent treatment are expected.

It is believed that, during cathodic activation in the sulphuric acid solution containing nickel ions, nickel nuclei may be created through the thin aluminium oxide film on the surface, thereby forming anchor spots for the nickel layer which is applied in a subsequent plating step. Thus, the cathodic activation step has the same effect as the creation of a thin bonding layer between the aluminium surface and the nickel coating. The same applies mutatis mutandis for the situation where iron or cobalt is being used.

The pre-treatment bath preferably contains about 15 to 200 g/l, and preferably 80 to 150 g/l, of NiSO₄.H₂O, and about 50 to 350 g/l, preferably about 150 to 250 g/l of H₂SO₄. In a preferred embodiment, the pre-treatment bath also contains boric acid as a buffer, e.g. in a range of 1 to 50 g/l, and preferably 20 to 40 g/l.

The preferred bath for pure nickel plating is a Watts bath containing nickel sulphate, nickel chloride and boric acid.

A preferred bath for nickel-bismuth plating is a citrate-gluconate bath containing nickel sulphate, nickel chloride, (NH₄)₂SO₄, bismuth concentrate, sodium citrate and sodium gluconate. Preferred concentration ranges of these substances are 100 to 180 g/l of NiSO₄.6H₂O, 10 to 50 g/l of NiCl₂.6H₂O, 1 to 10 ml/l of a bismuth concentrate containing 100 g/l of Bi, 10 to 50 g/l of (NH₄)₂SO₄, 100 to 180 g/l of sodium citrate.2H₂O, and 10 to 50 g/l of sodium gluconate. This bath may be used for pure nickel plating as well, in which case the bismuth concentrate is omitted.

It has been found that the pre-treatment is already effective at elevated temperatures of from above room temperature to less than, or up to, 95° C., and preferably in the range of 55° C. to 80° C. This is a great advantage, since working at lower temperatures makes the introduction into a strip plating line much easier, because evaporation losses will be limited. Furthermore, aluminium dissolution is much lower at temperatures below 70° C., thereby increasing the lifetime of the activation bath. Hence, the pre-treatment bath is preferably maintained at temperatures between 55° C. and 80° C., and most preferred between about 60° C. and 70° C.

The activation current is cathodic. As demonstrated by the examples, the current density is not critical to the quality of the final product. The same applies to activation time of the product in the pre-treatment bath. The activation current of the cathodic activation is preferably in a range of −200 to −2000 A/Nm², and more preferably in a range of −500 to −1400 A/m². The time spent by the product in the pre-treatment bath is typically in the range of 1 to 50 sec., and preferably in the range of 5 to 15 sec.

The average thickness of the applied metal layer of Ni, Co, Fe or alloys of each of these metals, is preferably less than 2 μm, more preferably less than 1.0 μm, and even more preferably in a range of 0.2 to 1.0 μm.

The method is preferably carried out as a continuous plating operation, which allows the continuous treatment of an infinite strip of metal.

In an optional additional step a further metal layer may be applied on top of the layer of Ni, Fe, Co, or alloys thereof, in order to improve for example the corrosion resistance of the final product. For example a thin layer of tin can be applied onto the nickel-layer on a brazing sheet product, which results in a significant improvement of the post-braze corrosion resistance.

The method according to this invention may include the additional step of degreasing of the surface prior to the cathodic activation and/or the electroplating step in order the clean the surface.

To avoid work hardening of soft annealed coils while processing them in a (vertical) plating line, it is advantageous to plate full hard material. Moreover, full hard material is easier to slit than soft annealed material. Thus, it is preferred to plate wide coils in full hard condition and split them afterwards into multiple coils of desired width, thereby reducing conversion costs. The coils may be soft annealed afterwards.

In an embodiment of the method according to the invention the aluminium workpiece is a brazing sheet product, the brazing sheet product including a core layer and a clad layer formed of a brazing alloy including aluminium and 2-18 wt. % silicon, preferably in the range of 7 to 14%, (such as AA4343 and 4045 alloys), and whereby the metal layer is applied on the clad layer. The metal layer of nickel, iron, cobalt or alloys of each of these metals act as a braze-promoting element during brazing.

In a preferred embodiment of the brazing sheet product the clad layer further comprises a wetting agent as alloying element in a range of up to 1 wt. % in order to improve the wettability of the clad alloy during the brazing process. And preferably the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, thallium and any mixture thereof.

In another embodiment of the method according to the invention the aluminium workpiece is an aluminium conductor, and preferably made of an alloy selected from the group consisting of AA1370, AA1110 and AA6101. The aluminium conductor can be in the form of an aluminium strip or aluminium wire or aluminium tube. For the embodiment the applied metal layer is preferably consisting of nickel in order to improve the electrical contact properties. The aluminium conductors can be used for the transmission of electrical and/or thermal energy. These conductors are usually in the form of bars, wire or cables when used as electrical conductors, and in the form of strips, bars or tubes when used as thermal conductors.

In a further aspect of the invention there is provided an aluminium alloy product, preferably a brazing sheet product, electroplated with a metal layer selected from the group consisting of nickel, iron, cobalt and alloys thereof manufactured with the method of the invention as set out in the present specification and claims. Such a brazing sheet product can be applied successfully in a Controlled Atmosphere Brazing (“CAB”) process in the absence of a brazing flux.

As shown by the following examples, the aluminium alloy product according to the invention has an excellently adhering nickel or nickel-bismuth coating. In a particularly preferred embodiment, the product is an aluminium alloy brazing sheet comprising a core, a clad layer and a nickel-containing layer plated on top of the clad layer. This brazing sheet will have good brazeability and low manufacturing costs. It may either contain a wetting agent like Bi in the clad alloy, or in the nickel-containing layer.

The following non-limiting examples illustrate the invention.

EXAMPLES

Two different types aluminium brazing sheets products of 0.4 mm thickness have been used for plating with a nickel or nickel-alloy layer having an average thickness of 0.5 μm. The aluminium brazing sheets used consisted of an AA3003-series aluminium core alloy conventionally clad on both sides with an AlSi brazing alloy, whereby clad layer A contained, in wt. %, 10% Si, 1.5% Mg and 0.08% Bi, whereas clad layer B contained, in wt. %, 12% Si and no Mg or Bi.

In producing nickel plated brazing sheet products the following procedure has been used:

Cleaning for 180 sec. at 50° C. using 35 g/l ChemTec 30014 (a commercial available bath), followed by rinsing;

Activation using a current density of −1000A/m², followed by rinsing;

Ni or Ni-Bi plating using a current density of −1000A/m², followed by rinsing.

The cathodic activation bath in accordance with the invention was prepared on basis of sulphuric acid (see Table 1). Nickel sulphate was selected to supply nickel-ions to the solution, and preferably boric acid was added as buffer. As an alternative a fluoride based activation bath was used (see Table 2) and consisting of anodic activation at a current density of +1000A/m², and which is disclosed in U.S. Pat. No. 6,780,303 B2, incorporated herein by reference. The cathodic activation was carried at various temperatures. Two Samples 10 and 11 have been carried out using the same activation bath but whereby the current was reversed such that anodic activation occurred.

After activation, either a nickel layer was plated from a Watts bath (see Table 3) or a nickel-bismuth alloy layer from a citrate-gluconate bath (see Table 4).

The quality of the resulting plated substrates were evaluated using an adhesion test and a brazeability test. The adhesion tests consisted on the Erichsen dome test (cup height of 5 mm), whereafter adhesive tape (Scotch Tape 3M No. 610) is applied to the deformed area and pulled off in one move. Adhesion is quantified by classifying the amount of nickel on the tape. An overall adhesion assessment was rated from 1 (poor) to 10 (excellent), wherein a level of 6 was considered acceptable as it was comparable to existing commercially available brazing sheet with a Ni-Pb layer.

On a laboratory scale of testing the brazing tests were carried out in a small quartz furnace. Small coupons of 25 mm×25 mm were cut out of the nickel-bismuth-plated sheets. A small strip of a bare AA3003 alloy measuring 30 mm×7 mm×1 mm was bent in the centre to an angle of 45° and laid on the coupons (see FIG. 1). The angle-on-coupon samples were heated under flowing nitrogen, with heating from room temperature to 580° C., dwell time at 580° C. for 1 minute, cooling from 580° C. to room temperature. The brazing process was judged on possible formation of wrinkles, capillary depression and fillet formation. An overall assessment was given where: (−)=poor brazeability, (±)=fair brazeability, and (+)=good brazeability.

The results of the experiments carried out and the adhesion and brazing performance for the various samples are summarized in Table 5. TABLE 1 Composition of the cathodic activation bath NiSO₄.6H₂O 100 g/l 96% H₂SO₄ (S.G. 1.84 kg l⁻¹) 200 g/l 113.2 ml/l H₃BO₃ 30 g/l

TABLE 2 Composition of the hydrofluoric acid bath NiCl₂.6H₂O 125 g/l 40% HF (S.G. 1.13 kg l⁻¹) 2.7 g/l 6 ml/l H₃BO₃ 12.5 g/l

TABLE 3 Composition of the Watts bath NiSO₄.6H₂O 270 g/l NiCl₂.6H₂O 50 g/l H₃BO₃ 30 g/l

TABLE 4 Composition of the citrate-gluconate bath NiSO₄.6H₂O 142 g/l NiCl₂.6H₂O 30 g/l (NH₄)₂SO₄ 34 g/l Na-citrate.2H₂O 140 g/l Na-gluconate 30 g/l bismuth concentrate (100 g/l Bi) 5 ml/l

TABLE 5 Summary of the experiments carried and the results on adhesion and brazeability Activation Plating Clad Activation Temperature Ni/ Braze- Sample layer bath (° C.) Ni—Bi Adhesion ability 1 B H₂SO₄ 70 Ni—Bi 5 ± 2 B H₂SO₄ 93 Ni—Bi 4 ± 3 A H₂SO₄ 50 Ni 6 +  4* A HF 50 Ni 9 + 5 A none — Ni 1 − 6 A H₂SO₄ 45 Ni 4 + 7 A H₂SO₄ 60 Ni 9 + 8 A H₂SO₄ 70 Ni 10 + 9 A H₂SO₄ 93 Ni 9 + 10* A H₂SO₄ 70 Ni 9 − 11* A H₂SO₄ 93 Ni 10 − *Samples 4, 10 and 11 involved anodic activation instead of cathodic activation as per Samples 1 to 3 and 6 to 9.

From the comparison of Samples 3, 4 and 5 it can be seen that if no activation is used both the adhesion and the brazeability is poor. Whereas a hydrofluoric acid bath obtains good adhesion and good brazeability, comparable to a sulphuric acid bath. However, a hydrofluoric acid bath contains fluoride and is therefore not the environmentally preferred method.

From examples 10 and 11 it can be seen that anodic activation provides excellent adhesion, but brazeability is seriously undermined possibly due to the formation of an oxide film. In accordance with the invention it has been found that both good adhesion and brazeability were obtained by cathodic activation. However, for those applications were brazeability is not required such as with aluminium conductors it might be a valuable pre-treatment method.

The temperature of the cathodic activation bath appeared to have a strong influence on the adhesion of the plated Ni layer. Sample 7 shows that adhesion and brazeability are still excellent at temperatures of about 60° C. However, if the temperature is lowered further to below 50° C. (Sample 6), the adhesion level becomes unacceptable. From Samples 1 and 2 and 8 and 9 it can be seen that neither adhesion nor brazeability suffers when the temperature is lowered from 93° C. to 70° C. This makes introduction into a continuous strip plating line much easier, because evaporation losses will be limited. Furthermore, aluminium dissolution is much less at 70° C. or lower, thereby increasing the lifetime of the activation bath.

The addition of a wetting agent such as Bi is favourable for the brazeability performance of the resultant brazing sheet product. From the Samples 1 and 8 it can be seen that the wetting agent might be added either to the Ni layer or to the brazing clad layer without affecting the adhesion or the brazeability. Adding the wetting agent to both the clad layer and the nickel layer has no adverse effect on the brazeability.

Thus, it has been demonstrated that direct nickel plating of a brazing sheet product is possible after cathodic activation in a simple sulphuric acid solution to which only nickel sulphate is added. No fluoride is needed in this activation process. Because the activation bath contains the same ingredients as a Watts bath, cross-contamination is excluded. Also, no problems in effluent treatment are expected. Satisfying results are obtained at bath temperatures of about 60 ° C. The process can be operated in a reliable manner over a wide range of current densities and treatments times.

It is believed that similar results will be obtained where iron or cobalt instead of nickel is used as braze-promoting metal on the brazing sheet product.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described. 

1. A method of applying a metal layer onto at least one surface of an aluminium or aluminium alloy workpiece, comprising the steps of pre-treating the surface by cathodic activation in a pre-treatment bath containing sulphuric acid and metal-ions selected from the group consisting of nickel, iron and cobalt, and applying a metal layer by electroplating the pre-treated workpiece, and wherein the metal layer is selected from the group consisting of nickel, iron, cobalt, and alloys thereof.
 2. The method according to claim 1, wherein the pre-treatment bath comprises 15 to 200 g/l of NiSO₄.H₂O and 50 to 350 g/l of H₂SO_(4.)
 3. The method according to claim 1, wherein the pre-treatment bath comprises 80 to 150 g/l of NiSO₄.H₂O and 150 to 250 g/l of H₂SO_(4.)
 4. The method according to claim 1, wherein the pre-treatment bath further comprises boric acid (H₃BO₃).
 5. The method according to claim 1, wherein the pre-treatment bath further comprises 1 to 50 g/l boric acid (H₃BO₃).
 6. The method according to claim 1, wherein the pre-treatment bath further comprises 20 to 40 g/l boric acid (H₃BO₃).
 7. The method according to claim 1, wherein the plating bath comprises a Watts bath.
 8. The method according to any one of claims 1, wherein the plating bath comprises a Watts bath comprising a mixture of nickel sulphate (NiSO₄), nickel chloride (NiCl₂) and boric acid (H₃BO₃).
 9. The method according to claim 1, wherein the plating bath comprises a citrate-gluconate bath comprising a mixture of NiSO₄, and (NH₄)₂SO4 and sodium citrate and sodium gluconate.
 10. The method according to claim 1, wherein the plating bath comprises a citrate-gluconate bath comprising a mixture of NiSO₄, nickel chloride, (NH₄)₂SO₄, bismuth concentrate, sodium citrate and sodium gluconate.
 11. The method according to claim 1, wherein the plating bath comprises a citrate-gluconate bath comprising a mixture of 100 to 180 g/l of NiSO₄.6H₂O, 10 to 50 g/l of NiCl₂.6H₂O, 1 to 10 ml/l of a bismuth concentrate containing 100 g/l of Bi, 10 to 50 g/l of (NH₄)₂SO₄, 100 to 180 g/l of sodium citrate.2H₂O, and 10 to 50 g/l of sodium gluconate.
 12. The method according to claim 1, wherein the plating bath comprises a citrate-gluconate bath comprising a mixture of NiSO₄, nickel chloride, (NH₄)₂SO₄, sodium citrate and sodium gluconate.
 13. The method according to claim 1, wherein the plating bath comprises a citrate-gluconate bath comprising a mixture of 100 to 180 g/l of NiSO₄.6H₂O, 10 to 50 g/l of NiCl₂.6H₂O, 10 to 50 g/l of (NH₄)₂SO₄, 100 to 180 g/l of sodium citrate.2H₂O, and 10 to 50 g/l of sodium gluconate.
 14. The method according to claim 1, wherein the pre-treatment bath is devoid of any fluoride containing components.
 15. The method according to claim 1, wherein the temperature of the pre-treatment bath is maintained at an elevated temperature in a range of above room temperature to 95° C.
 16. The method according to claim 1, wherein the temperature of the pre-treatment bath is maintained at a temperature in a range of 55° C. to 80° C.
 17. The method according to claim 1, wherein the temperature of the pre-treatment bath is maintained at a temperature in a range of between about 60° C. and 70° C.
 18. The method according to claim 1, wherein the cathodic activation applies an activation current in a range of −200 to −2000 A/m²,
 19. The method according to claim 1, wherein the cathodic activation applies an activation current in a range of −500 to −1400 A/m².
 20. The method according to claim 1, wherein the time spent by the product in the pre-treatment bath is in the range of 1 to 50 sec.
 21. The method according to claim 1, wherein the time spent by the product in the pre-treatment bath is in the range of 5 to 15 sec.
 22. The method according to claim 1, wherein the applied metal layer has an average thickness of less than 2 μm.
 23. The method according to claim 1, wherein the applied metal layer has an average thickness of less than 1.0 μm.
 24. The method according to claim 1, wherein the applied metal layer has an average thickness of 0.2 to 1.0 μm.
 25. The method according to claim 1, further comprising applying a second metal layer on top of the layer of Ni, Fe, Co, or alloys thereof.
 26. The method according to claim 1, wherein the layer of Ni, Fe, Co, or alloys thereof is a nickel-containing layer, further comprising applying a tin-containing layer on top of the nickel-containing layer.
 27. The method according to claim 1, wherein the method is carried out as a continuous plating operation.
 28. The method according to claim 1, wherein the aluminium workpiece is a brazing sheet product, the brazing sheet product including a core layer and a clad layer formed of a brazing alloy including aluminium and 2-18 wt. % silicon, and whereby the metal layer is applied on the clad layer.
 29. The method according to claim 28, wherein the clad layer further comprises as alloying element a wetting agent in a range of up to 1 wt. %.
 30. The method according to claim 29, wherein the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, thallium and any mixture thereof.
 31. The method according to claim 1, wherein the aluminium workpiece is an aluminium conductor, and preferably made of an alloy selected from the group consisting of AA1370, AA1110 and AA6101.
 32. An aluminium alloy product, electrolytically plated with a metal layer selected from the group consisting of nickel, iron, cobalt and alloys thereof by using the method of claim
 1. 